See first video, world 4 part 1, and world 7 part 1 for more details.
Each of the doors was recorded separately, plus savestates WERE used, BETWEEN each door to conveniently get all of them in one fell swoop.
0:01 King Boo's Castle: This is it! King Boo's Castle! There are 9 rooms (grouped in 3 groups of 3), from which you must do only three, though I did all nine for the sake of showing you all nine. The challenges in each room vary greatly. For the second half, you have to climb up the castle, pass through a hallway with many Boos and Eeries while dealing with a moving layer, climb up a bit more, then fight the hack's ultimate final boss! Do this, and you've beaten the hack.
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